This invention relates to a film having improved low birefringence, to a multilayer film comprising a layer of such film, and to methods and optical devices for making and using such films.
Use of optical storage devices has become common since the advent of the compact disc (CD) widely used for the storage of music, video and other information. Optical storage devices of this type require transparent substrates with excellent optical properties. A substrate is encoded with information often by molding in a series of pits or depressions. Suitably coated this substrate can be read by a laser to give a series of signals recovering the information stored on the disc. With storage devices of this type, there is a growing need to store more and more information in a smaller space.
Thermoplastic polymers such as Bisphenol A Polycarbonate (BPA-PC) have been widely used for optical storage media applications, however, thermo plastics such as BPA-PC have some limitations. It is rather difficult to process thermoplastics in an extrusion film process and meet the optical requirements for optical storage media. In these applications thermoplastic polymers such as BPA-PC and optical data storage devices made from them are limited by their birefringence. Birefringence, resulting from the inherent properties of the resin and also from how it was processed (influenced by its rheological properties) can interfere with the recovery of information stored on the device (i.e. disc).
In the further development of optical discs, particularly read-write discs and discs which are capable of storing larger amounts of data, various physical factors become increasingly important. Again, one such factor, which is closely related to the storage capacity of the disc, is birefringence, i.e., the difference between indices of refraction for light polarized in perpendicular directions. Birefringence leads to phase retardation between different polarization components of the laser beam (i.e., a polarization-dependent effect), thereby reducing readability of the disc. Therefore thermoplastic films having low birefringence, typically not more than 1xc3x9710xe2x88x924 nm/nm, are desirable components for optical discs.
Other applications for films with low birefringence, typically not more than 1xc3x9710xe2x88x924 nm/nm, are polarizers for liquid crystal displays.
Birefringence has several sources including the physical processing of the raw material from which films are fabricated, the degree of molecular orientation therein, and thermal stresses in a fabricated film. The observed birefringence of a film is therefore determined by the molecular structure, which determines the intrinsic birefringence, and the processing conditions, which can create thermal stresses and orientation of the polymer chains. Specifically, the observed birefringence is typically a function of the intrinsic birefringence plus the birefringence introduced upon fabricating articles such as film substrates.
Shear stress introduced at the wall of extrusion dies used to form thermoplastic films such as BPA-PC result in highly oriented molecular structure of the film in the proximity of the outer surfaces of such films. This oriented molecular structure results in much higher birefringence of the film near the surfaces and thus high total birefringence of the bulk film.
It has been common practice to prepare low birefringence films using solvent-borne film coating materials in which stresses are relieved through the slow evaporation of the solvents. However, such processes are undesirable from the emissions standpoint and from the standpoint of the presence of entrained solvents in the polymeric film. Solvent cast, e.g. polycarbonate, sheet can be made with relatively low birefringence, but processing speeds are very slow (and expensive). The samples retain excessive levels of residual casting solvent. Standard molecular weight polycarbonate can also crystallize from solution instead of casting a clear (mostly amorphous) sheet.
It has been disclosed in U.S. Pat. No. 4,617,207 that by co-extruding a thermoplastic resin sheet in the core of two non-adhesive peelable resins that the shear stress introduced at the wall of extrusion dies used to form the thermoplastic film is reduced resulting in films with low double refractive index or birefringence. However, thermoplastic films of desirable materials for optical read and write storage medias, such as polycarbonate, polyester, polystyrene, and poly(methylmethacrylate) were not demonstrated at the desirable thicknesses (less than 250 xcexcm) for such applications in U.S. Pat. No. 4,617,207. It has been shown that only under special conditions can such films of the desired thickness and birefringence be produced. Such films also have utility as polarizers for liquid crystal displays where again, films of less than 250 xcexcm are desirable.
Therefore, there is a need to be able to prepare extruded thermoplastic transparent melt processable films of thickness less than 250 xcexcm that exhibit a birefringence of less than 1xc3x9710xe2x88x924 nm/nm.
The invention provides a coextruded film comprising a transparent polymeric inner core layer of thickness less than 250 xcexcm and two peelable outer polymeric layers wherein the inner layer constitutes a low enough proportion, less than 40%, of the total film thickness and exhibits a low enough melt flow index value to achieve a birefringence of not more than 1xc3x9710xe2x88x924 nm/nm. The invention also provides such a film bearing one peelable layer, the extruded inner layer film, and a process and optical devices for making and using such films.
The invention provides low birefringence films that have a birefringence of not more than 1xc3x9710xe2x88x924 nm/nm, have no residual solvent, and are less than 250 xcexcm thick.